1. Field of the Invention
The present invention relates to a method for enabling a terminal to periodically and/or aperiodically report channel state information to a base station, and a method for enabling the base station to receive the channel state information, wherein it is assumed that the channel state information for use in a multi-antenna system (i.e., a Multiple-Input Multiple-Output (MIMO) system) includes a channel quality indicator (CQI), a preceding matrix index (PMI), and a rank indicator (RI).
2. Discussion of the Related Art
Generally, a multi-antenna technology (hereinafter referred to as an MIMO technology) will hereinafter be described in detail.
In brief, the MIMO technology is an abbreviation of the Multi-Input Multi-Output technology. The MIMO technology uses multiple transmission (Tx) antennas and multiple reception (Rx) antennas to improve the efficiency of transmission/reception (Tx/Rx) data, whereas a conventional art has generally used a single transmission (Tx) antenna and a single reception (Rx) antenna. In more detail, the MIMO technology is not dependent on a single antenna path to receive a single entire message, and completes the entire message by collecting a plurality of data fragments received via several antennas. As a result, the MIMO technology can increase a data transfer rate within a specific range, or can increase a system range at a specific data transfer rate.
FIG. 1 is a block diagram illustrating a conventional MIMO system.
If the number of antennas of a reception end and the number of antennas of a reception end are simultaneously increased as shown in FIG. 1, theoretical channel transmission capacity increases in proportion to the number of antennas in a different way from a conventional art in which only a transmitter or a receiver uses multiple antennas, such that a frequency efficiency can be greatly improved.
The MIMO technology can be classified into a spatial diversity scheme and a spatial multiplexing scheme. The spatial diversity scheme increases transmission reliability using symbols passing various channel paths. The spatial multiplexing scheme simultaneously transmits a plurality of data symbols via a plurality of Tx antennas, so that it increases a transfer rate of data. In addition, the combination of the spatial diversity scheme and the spatial multiplexing scheme has also been recently researched to properly acquire unique advantages of the two schemes.
Detailed descriptions of the spatial diversity scheme, the spatial multiplexing scheme, and the combination thereof will hereinafter be described in detail.
Firstly, the spatial diversity scheme will hereinafter be described. The spatial diversity scheme is classified into a space-time block code scheme and a space-time Trellis code scheme which simultaneously uses a diversity gain and a coding gain. Generally, a bit error ratio (BER) improvement performance and a code-generation degree of freedom of the space-time Trellis code scheme are superior to those of the space-time block code scheme, whereas the calculation complexity of the space-time block code scheme is superior to that of the space-time Trellis code scheme. A spatial diversity gain corresponds to the product or multiplication of the number of Tx antennas and the number of Rx antennas. In the meantime, if a space-time coding scheme is applied to a frequency domain instead of a time domain, this space-time coding scheme may also be considered to be a frequency-space coding scheme, and a coding scheme applied to this frequency-space coding scheme is equal to that of the space-time coding scheme.
Secondly, the spatial multiplexing scheme will hereinafter be described. The spatial multiplexing scheme is adapted to transmit different data streams via individual Tx antennas. In this case, a receiver may unavoidably generate mutual interference between data fragments which have been simultaneously transmitted from a transmitter. The receiver removes this mutual interference using a proper signal processing technique, so that it can receive the resultant data having no interference. In order to remove noise from the received data, a maximum likelihood receiver, a ZF receiver, a MMSE receiver, a D-BLAST, or a V-BLAST may be used. Specifically, if a transmission end is able to recognize channel information, a Singular Value Decomposition (SVD) scheme may be used to remove the noise.
Thirdly, the combination of the spatial diversity scheme and the spatial multiplexing scheme will hereinafter be described. Provided that only a spatial diversity gain is acquired, a performance-improvement gain is gradually saturated in proportion to an increasing diversity order. Otherwise, provided that only the spatial multiplexing gain is acquired, a transmission reliability of a radio frequency (RF) channel is gradually deteriorated. As a result, a variety of schemes capable of acquiring both the aforementioned two gains simultaneously while solving the above-mentioned problems have been intensively researched by many companies or developers, for example, a double-STTD scheme and a space-time BICM (STBICM) scheme.
In the meantime, a general communication system performs coding of transmission information of a transmission end using a forward error correction code, and transmits the coded information, so that channel errors can be corrected by a reception end. The reception end demodulates a received (Rx) signal, performs decoding of forward error correction code, and recovers transmission information. By the decoding process, errors of the Rx signal caused by the channel can be corrected.
Each of all forward error correction codes has a maximum-correctable limitation in a channel error correction. In other words, if a reception (Rx) signal has an error exceeding the limitation of a corresponding forward error correction code, a reception end is unable to decode the Rx signal into information having no error. Therefore, there is a need for the reception end to determine the presence or absence of an error in the decoded information. In this way, a specialized coding process for performing error detection is required, separately from the forward error correction coding process. Generally, a Cyclic Redundancy Check (CRC) code has been used as an error detection code.
The CRC method is an exemplary coding method for performing the error detection instead of the error correction. Generally, transmission information is coded by the CRC method, and then the forward error correction code is applied to the CRC-coded information. A single unit coded by the CRC and the forward error correction code is generally called a codeword. Respective codewords are mapped to streams corresponding to ranks, and the mapped result is transmitted, where the number of streams is equal to the number of ranks corresponding to independent channels of a MIMO communication system.
Meanwhile, in the above-mentioned MIMO system, a transmission end performs preceding of Tx data, and transmits the precoded Tx data, and a reception end receives signals using a preceding vector used by the transmission end.
The preceding vector for performing the above preceding is set to any one of preceding vectors which have been predefined as a codebook format in transmission/reception ends. In this case, a transmission scheme of the transmission end can be classified into an open-loop transmission method and a closed-loop transmission method according to specific information indicating whether or not the preceding vector of the transmission end requests feedback information from the reception end.
In case of the open-loop transmission method, the transmission end selects a preceding vector without using feedback information of the reception end, and transmits signals. Otherwise, in case of the closed-loop transmission method, the reception end indicates a specific preceding vector among predefined codebooks according to a reception end, and feeds back channel information associated with the specific preceding vector, such that the transmission end transmits signals using such a feedback signal.
In the meantime, in order to implement effective communication, there is a need for channel information to be notified along a feedback path, downlink channel information is uploaded to an uplink, and uplink channel information is downloaded to a downlink. The downlink or uplink channel information is represented by a channel information indicator (CQI), i.e., a channel quality indicator (CQI). This CQI can be generated by various methods. For example, a method for quantizing channel state information without any change and uploading the quantized channel state information, a method for calculating an SINR and uploading the calculated SINR, and a method for notifying a channel's actual application state as in a modulation coding scheme (MCS) can be used.
In this MIMO system, channel state information (CSI) to be notified to a base station by a terminal may include the above CQI, the preceding matrix index (PMI), and the rank indicator (RI) indicating the number of independent channels. This channel state information is periodically notified to a base station over a Physical Uplink Control Channel (PUCCH) and/or a Physical Uplink Shared Channel (PUSCH).